Down-hole seismic sources are used in geophysical prospection techniques, such as cross-hole seismic tomography and up-hole seismic tomography, in order to perform energizations within exploration boreholes.
In-hole seismic prospections allow to calculate the velocity of artificially generated seismic waves and thus to estimate the elastic parameters which are characteristic of the materials that constitute the subsoil, by executing in-hole energizations at different depths and recording the times of arrival of the wavefronts, still at different depths, into one or more holes and/or at multiple points of a clear surface, e.g., the surface of the ground, a slope, or a face. More specifically, the times acquired and the known distances allow to build a distribution map of the values of the speed of propagation of the elastic waves (cross-hole or up-hole seismic tomography, etc.).
Until recently, the only way of executing adequately powerful energizations within an exploration borehole was to use microcharges of explosive material. This practice has now been almost entirely abandoned, at least in the geotechnical field, because of its high costs and of the highly restrictive rules that regulate the use and transportation of explosive materials; as a replacement, alternative seismic sources have been developed which are safer, more economical, and capable of providing high-quality signals.
Said alternative seismic sources are essentially of two types:
compressed-air sources (exploiting the energy associated with the quick escape of pressurized air through suitable nozzles);
sparker-type sources (wherein energy is generated by instantaneous vaporization of a given volume of water caused by an electric arc).
Their use, however, requires the availability of power generators, compressors, hydraulic systems, electric systems with capacitors, and is therefore often uncomfortable, especially in impervious areas.
It must also be pointed out that scientific literature directly concerning the use of these energy sources as well as the advantages and problems thereof is virtually inexistent, since energization is essentially a technical means.
There is therefore a need to provide more effective seismic sources which are easy to use, economical, safe, and not subject to particular restrictions.